This application claims priority to Japanese application No. 2000-330966, filed Oct. 30, 2000, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a distributed optical amplifying apparatus which can serve both as an optical transmission line and an optical amplifying medium, and more particularly, to a distributed optical amplifying apparatus which can compensate transmission loss, prevent a nonlinear optical effect, and improve an optical signal-to-noise ratio. Moreover, the present invention relates to an optical fiber cable suitable for the distributed optical amplifying apparatus, an optical communication station including the distributed optical amplifying apparatus, and an optical communication system including the distributed optical amplifying apparatus.
2. Description of the Related Art
Optical communication systems will be used in future multimedia networks, as advances in optical communication technology should enable the high bandwidth, high capacity, ultra long distance transmission required by such future multimedia networks. Wavelength division multiplexing (hereinafter abbreviated to xe2x80x98WDMxe2x80x99) is a significant optical communication technology being developed for this purpose, as WDM effectively utilizes the broadband characteristics and large capacity of an optical fiber.
More specifically, in WDM optical communication systems, a plurality of optical signals at different wavelengths are multiplexed together into a WDM optical signal. This WDM optical signal is then transmitted through a single optical fiber as an optical transmission line. A WDM optical communication system can provide extremely high bandwidth, high capacity, long distance transmission.
In a long distance optical communication system, since a WDM optical signal attenuates while being transmitted through an optical transmission line, the WDM optical signal must be amplified after being transmitted a certain distance. For this reason, optical amplifying apparatuses for amplifying the WDM optical signal are currently in use, and further research and development of such apparatuses is occurring.
Therefore, in a conventional WDM optical communication system, an optical transmitting station uses wavelength division multiplexing to multiplex together a plurality of optical signals at different wavelengths into a WDM optical signal. The WDM optical signal is then transmitted through an optical transmission line. An optical receiving station receives the transmitted WDM optical signal from the optical transmission line. One or more optical repeater stations are positioned along the optical transmission line to amplify the WDM optical signal. The number of optical repeater stations is typically determined in accordance with system design parameters to provide a sufficient amount of amplification.
While being transmitted through the optical transmission line, the WDM optical signal deteriorates in its waveform due to wavelength dispersion, transmission loss, and a nonlinear optical effect. Therefore, various countermeasures have been devised.
For example, various conventional methods have been devised for providing wavelength dispersion compensation. In one such method, a dispersion-managed fiber (hereinafter abbreviated to xe2x80x98DMFxe2x80x99) combines optical fibers with different wavelength dispersion from each other.
FIGS. 1A and 1B are diagrams showing the structures of conventional dispersion-managed fibers.
More specifically, FIG. 1A shows a partial structure between two stations in an optical communication system, where an optical repeater station 1004-A and an optical repeater station 1004-B are connected by an optical transmission line 1002. The optical transmission line 1002 is composed of an optical transmission line 1002-L1 whose wavelength dispersion is positive and an optical transmission line 1002-L2 whose wavelength dispersion is negative. An optical signal is transmitted to the optical repeater station 1004-B from the optical repeater station 1004-A via the optical transmission line 1002-L1 and the optical transmission line 1002-L2. While being transmitted, the optical signal undergoes a positive wavelength dispersion in the optical transmission line 1002-L1 and undergoes a negative wavelength dispersion in the optical transmission line 1002-L2 to be compensated in a manner that accumulated wavelength dispersion becomes almost zero. The DMF as described above is disclosed, for example, in U.S. Pat. No. 5,191,631, and Japanese Patent Laid-open No. Hei 9-318824. A symmetrical DMF in which the wavelength dispersion is made symmetrical is also disclosed.
FIG. 1B shows a partial structure of two stations in an optical communication system. An optical repeater station 1004-C and an optical repeater station 1004-D are connected by the optical transmission line 1002. The optical transmission line 1002 is composed of an optical transmission line 1002-L3 whose wavelength dispersion is positive, an optical transmission line 1002-L4 whose wavelength dispersion is negative, and an optical transmission line 1002-L5 whose wavelength dispersion is positive. An optical signal which is sent out from the optical repeater station 1004-C undergoes the positive wavelength dispersion in the optical transmission line 1002-L3, undergoes the negative wavelength dispersion in the optical transmission line 1002-L4, and undergoes the positive wavelength dispersion in the optical transmission line 1002-L5. Therefore, the optical signal is transmitted to the optical repeater station 1004-D in a manner so that compensation causes accumulated wavelength dispersion to become almost zero. Meanwhile, an optical signal sent out from the optical repeater station 1004-D undergoes the positive wavelength dispersion in the optical transmission line 1002-L5, undergoes the negative wavelength dispersion in the optical transmission line 1002-L4, and undergoes the positive wavelength dispersion in the optical transmission line 1002-L3. Therefore, the optical signal is transmitted to the optical repeater station 1004-C in a manner so that compensation causes accumulated wavelength dispersion to become almost zero. Such a DMF is disclosed, for example, in U.S. Pat. No. 5,778,128, a paper, xe2x80x9cEnhanced power solitons in optical fibers with periodic dispersion managementxe2x80x9d (N. J. Smith, F. M. Knox, N. J. Doran, K. J. Blow and I. Bennion: Electronics Letters, Vol. 31, No. 1, p54-p55, Jan. 4, 1996), a paper, xe2x80x9cEnergy-scaling characteristics of solitons in strongly dispersion-managed fibersxe2x80x9d (N. J. Smith, N. J. Doran, F. M. Knox and W. Forysak: Optics Letters, Vol. 21, No. 24, p1981-p1983, Dec. 15, 1966), and a paper, xe2x80x9c40 Gbit/sxc3x9716 WDM transmission over 2000 km using dispersion managed low-nonlinear fiber spanxe2x80x9d (Itsuro Morita, Keiji Tanaka, Noboru Edagawa and Masatoshi Suzuki: ECOC 2000, Vol. 4, p25-p26, 2000).
These conventional technologies are devised from the viewpoint of wavelength dispersion compensation. Such technologies were not devised in consideration of a system in which an optical transmission line also serves as an optical amplifying medium for distributed optical amplification.
Meanwhile, various methods for compensating the transmission loss have also been conventionally devised, and a distributed optical amplifying apparatus, especially a distributed Raman amplifier, is one of them.
FIGS. 2A and 2B are diagrams showing the structures of conventional loss compensated/distributed Raman amplifiers.
FIG. 2A shows a partial structure between two stations in the optical communication system described above, where the optical repeater station 1004-A and an optical repeater station 1004E are connected with the optical transmission line 1002. In the optical repeater station 1004-E, a pump light source 1005-E for supplying pump light used for Raman amplification is provided. The optical transmission line 1002 is composed of an optical transmission line 1002-L6 which has a large effective cross section and an optical transmission line 1002-L7 which has a small effective cross section compared with that of the optical transmission line 1002-L6, and it is supplied with the pump light from the pump light source 1005E. An optical signal is transmitted from the optical repeater station 1004-A to the optical repeater station 1004-E via the optical transmission line 1002-L6 and the optical transmission line 1007-L7, and is Raman-amplified by the pump light in the optical transmission line 1002 while being transmitted to be compensated in such a manner that transmission loss becomes almost zero. In other words, the optical signal is Raman-amplified so that an output optical level of the optical repeater station 1004-A and an input optical level of the optical repeater station 1004-E are substantially equal to each other. The effective cross section is a part of a cross section of the optical transmission line in which the optical signal and the pump light interact with each other to cause sufficient Raman amplification. Such a DMF is disclosed, for example, in a paper, xe2x80x9c40 Gbit/sxc3x978 NZR WDM transmission experiment over 80 kmxc3x975-span using distributed Raman amplification in RDFxe2x80x9d (R. Ohhira, Y. Yano, A. Noda, Y. Suzuki, C. Kurioka, M. Tachigori, S. Moribayashi, K. Fukuchi, T. Ono and T. Suzaki: ECOC xe2x80x299, 26-30, p176-p177, September 1999, Nice, France).
Here, a size of the effective cross section correlates with a scale of the nonlinear optical effect. When the effective cross section is large, the nonlinear optical effect is small. On the other hand, when the effective cross section is small, the nonlinear optical effect is large. Therefore, from the viewpoint of a choice of whether optical power in the optical repeater station 1004-A from which the optical signal is sent out is increased or optical power in the optical repeater station 1004-E from which the pump light is supplied is increased, a structure as shown in FIG. 2B in also possible. In FIG. 2B, the optical transmission line 1002 is composed of an optical transmission line 1002-L8 which has a small effective cross section and an optical transmission line 1002-L9 which has a large effective cross section compared with the optical transmission line 1002-L8. The optical transmission line 1002-L8 is connected to the optical repeater station 1004-A. Such a structure is disclosed, for example, in a paper, xe2x80x9cA proposal of a transmission line without any loss in a longitudinal direction utilizing distributed Raman amplificationxe2x80x9d (Toshiaki Okuno, Tetsufumi Tsuzaki and Masayuki Nishimura: B-10-116, the 2000 Society Conference of the Institute of Electronics, Information and Communication Engineers).
These conventional technologies as shown in FIGS. 2A and 2B are technologies which are devised from the viewpoint of compensating the transmission loss and no consideration is made for the wavelength dispersion compensation, an optical signal-to-noise ratio (hereinafter abbreviated to xe2x80x98optical SNRxe2x80x99), and so on.
Furthermore, in the conventional arts as shown in FIGS. 1A and 1B and FIGS. 2A and 2B, the nonlinear optical effect, especially a nonlinear phase shift, is not taken into consideration.
It is noteworthy that the wavelength dispersion and the effective cross section have such a correlation that an optical fiber with the positive wavelength dispersion usually has a small effective cross section and an optical fiber with the negative wavelength dispersion usually has a large effective cross section.
In realizing long distance transmission of an optical signal with less error ratio, there is a problem that the wavelength dispersion, the transmission loss, and the nonlinear optical effect need to be compensated in a well-balanced manner as a whole instead of compensating only one physical quantity.
It is an object of the present invention to provide an optical amplifying medium having appropriate characteristics for providing distributed optical amplification and which solves various of the above-described problems. It is also an object of the present invention to provide a distributed optical amplifying apparatus, an optical fiber cable, an optical communication station, and an optical communication system which use such an optical amplifying medium.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing a distributed optical amplifying apparatus, including an optical fiber having a middle field with a characteristic value which is larger than characteristic values of fields other than the middle field, the characteristic value of a respective field being a nonlinear refractive index of the optical fiber at the respective field divided by an effective cross section of the fiber at the respective field. The apparatus also includes a pump light source supplying pump light to the optical fiber.
Objects of the present invention are also achieved by providing a distributed optical amplifying apparatus including a fiber line and a pump light source supplying pump light to the fiber line. The fiber line includes first, second and third optical fibers connected together so that light travels through the fiber line from the first optical fiber, then through the second optical fiber and then through the third optical fiber. The first, second and third optical fibers having first, second and third characteristic values, respectively. The second characteristic value is larger than the first characteristic value and the third characteristic value. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber.
Objects of the present invention are further achieved by providing a distributed optical amplifying apparatus including (a) a fiber line comprising first, second and third optical fibers connected together so that light traveling through the fiber line travels through the first optical fiber, then through the second optical fiber, and then through the third optical fiber, and (b) a pump light source supplying pump light to the fiber line. A value D1/S1 of a wavelength dispersion coefficient D1 of the first optical fiber divided by a wavelength dispersion slope S1 thereof is almost equal to a value D2/S2 of a wavelength dispersion coefficient D2 of the second optical fiber divided by a wavelength dispersion slope S2 thereof. A sum of a value D1xc2x7L1 of the wavelength dispersion coefficient D1 of the first optical fiber multiplied by a length L1 thereof and a value of the wavelength dispersion coefficient D2 of the second optical fiber multiplied by a length L2 thereof is almost zero. A wavelength dispersion coefficient, a wavelength dispersion slope, and a length of the third optical fiber are almost equal to the wavelength dispersion coefficient D1, the wavelength dispersion slope S1, and the length L1 of the first optical fiber. Accumulated wavelength dispersion in a wavelength of an optical signal transmitted through the fiber line is almost zero at an output of the fiber line. An accumulated wavelength dispersion slope in the wavelength of the optical signal transmitted through the fiber line is almost zero at the output of the fiber line.
Objects of the present invention are also achieved by providing an optical communication station including a processing device for performing predetermined processing for an optical signal, and a fiber line connected to the processing device. The fiber line includes first, second and third optical fibers connected together so that the optical signal travels through the fiber line by traveling through the first optical fiber, then through the second optical fiber, and then through the third optical fiber. The first, second and third optical fibers have first, second and third characteristic values, respectively. The second characteristic value is larger than the first and third characteristic values. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. A pump light source supplies pump light to the fiber line.
Objects of the present invention are further achieved by providing an optical communication system including (a) an optical transmission line, (b) first and second stations connected together through the optical transmission line and performing predetermined processing of an optical signal transmitted through the optical transmission line, and (c) a pump light source supplying pump light to the transmission line. The transmission line includes first, second and third optical fibers connected together so that the optical signal travels through the transmission line by traveling through the first optical fiber, then through the second optical fiber, and then through the third optical fiber. The first, second and third optical fibers have first, second and third characteristic values, respectively. The second characteristic value being larger than the first and third characteristic values. The characteristic value of a respective optical fiber is a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber.
Moreover, objects of the present invention are achieved by providing an optical communication system including (a) first and second transmission lines, each having first and second ends, (b) an optical transmitting station generating an optical signal and providing the generated optical signal to the first end of the first transmission line so that the optical signal travels through the first transmission line to the second end of the first transmission line, (c) an optical repeater station receiving the optical signal from the second end of the first transmission line, amplifying the received optical signal, and providing the amplifying optical signal to the first end of the second transmission line so that the amplified optical signal travels through the second transmission line to the second end of the second transmission line, and (d) an optical receiving station receiving the amplified optical signal from the second end of the second optical transmission line. At least one of the first and second transmission lines includes first, second and third optical fibers connected together so that the optical signal travels through the respective transmission line by traveling through the first optical fiber, then through the second optical fiber, and then through the third optical fiber. The first, second and third optical fibers having first, second and third characteristic values, respectively, the second characteristic value being larger than the first and third characteristic values. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. Pump light source is providing to the respective transmission line.
Objects of the present invention are achieved by providing an optical fiber cable including a plurality of optical fibers. Each optical fiber has a characteristic value in a middle field which is larger than characteristic values in fields other than the middle field of the optical fiber, the characteristic value in a respective field being a nonlinear refractive index of the optical fiber in the field divided by an effective cross section of the optical fiber in the field.
Objects of the present invention are also achieved by providing an optical fiber cable including first, second and third optical fibers connected together so that light traveling through the cable travels through the first optical fiber, then through the second optical fiber and then through the third optical fiber. The second optical fiber has a negative dispersion value. The first and third optical fibers each have a positive dispersion value. The cable optically connects together two optical repeater stations, or an optical repeater station and an end station.
Moreover, objects of the present invention are achieved by providing an optical communication system including an optical fiber cable comprising first, second and third optical fibers connected together so that light traveling through the cable travels through the first optical fiber, then through the second optical fiber and then through the third optical fiber. The second optical fiber has a negative dispersion value. The first and third optical fibers each have a positive dispersion value. The cable optically connects together (a) two optical repeater stations with one of the optical repeater stations providing pump light to the cable so that distributed Raman amplification occurs in the cable, or (b) an optical repeater station and an end station so that one of the optical repeater station and the end station provides pump light to the cable so that distributed Raman amplification occurs in the cable.
In addition, objects of the present invention are achieved by providing an apparatus including a transmission line and a pump light source. The transmission line includes first, second and third optical fibers connected together from an input end to an output end of the transmission line so that a signal light travels through the input end, then through the first optical fiber, then through the second optical fiber, then through the third optical fiber and then through the output end. The first, second and third optical fibers have first, second and third characteristic values, respectively. The second characteristic value is larger than the first characteristic value and the third characteristic value. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. The pump light source supplies pump light to the transmission line so that the signal light is amplified by Raman amplification as the signal light travels through the transmission line
Further, objects of the present invention are achieved by providing an apparatus including a transmission line and a pump light source. The transmission line includes first, second and third optical fibers connected together from an input end to an output end of the transmission line so that a signal light travels through the input end, then through the first optical fiber, then through the second optical fiber, then through the third optical fiber and then through the output end. The pump light source supplies pump light to the transmission line so that the signal light is amplified by Raman amplification as the signal light travels through the transmission line. A value D1/S1 of a wavelength dispersion coefficient D1 of the first optical fiber divided by a wavelength dispersion slope S1 thereof is almost equal to a value D2/S2 of a wavelength dispersion coefficient D2 of the second optical fiber divided by a wavelength dispersion slope S2 thereof. A sum of a value D1xc2x7L1 of the wavelength dispersion coefficient D1 of the first optical fiber multiplied by a length L1 thereof and a value of the wavelength dispersion coefficient D2 of the second optical fiber multiplied by a length L2 thereof is almost zero. A wavelength dispersion coefficient, a wavelength dispersion slope, and a length of the third optical fiber are almost equal to the wavelength dispersion coefficient D1, the wavelength dispersion slope S1, and the length L1 of the first optical fiber. Accumulated wavelength dispersion in a wavelength of the signal light is almost zero at the output end of the transmission line. An accumulated wavelength dispersion slope in the wavelength of the signal light is almost zero at the output end of the transmission line.
Moreover, objects of the present invention are achieved by providing an optical communication system including (a) an optical transmission line, (b) first and second stations connected together through the optical transmission line and performing predetermined processing of a signal light transmitted through the transmission line, and (c) a pump light source supplying pump light to the transmission line so that the signal light is amplified by Raman amplification as the signal light travels through the transmission line. The transmission line includes first, second and third optical fibers connected together so that the optical signal travels through the transmission line by traveling through the first optical fiber, then through the second optical fiber, and then through the third optical fiber. The first, second and third optical fibers have first, second and third characteristic values, respectively, the second characteristic value being larger than the first and third characteristic values. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber.
Objects of the present invention are also achieved by providing an optical communication system including (a) first and second transmission lines, each having first and second ends, (b) an optical transmitting station generating an optical signal and providing the generated optical signal to the first end of the first transmission line so that the optical signal travels through the first transmission line to the second end of the first transmission line, (c) an optical repeater station receiving the optical signal from the second end of the first transmission line, amplifying the received optical signal, and providing the amplifying optical signal to the first end of the second transmission line so that the amplified optical signal travels through the second transmission line to the second end of the second transmission line, and (d) an optical receiving station receiving the amplified optical signal from the second end of the second optical transmission line. At least one of the first and second transmission lines includes first, second and third optical fibers connected together so that the optical signal travels through the respective transmission line by traveling through the first optical fiber, then through the second optical fiber, and then through the third optical fiber. The first, second and third optical fibers having first, second and third characteristic values, respectively, the second characteristic value being larger than the first and third characteristic values. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. Pump light being supplied to the respective transmission line so that the optical signal is amplified by Raman amplification as the optical signal travels through the respective transmission line.
Objects of the present invention are further achieved by providing an apparatus including a transmission line formed of first, second and third optical fibers, the first optical fiber being connected to the second optical fiber and the second optical fiber being connected to the third optical fiber so that a signal light traveling through the transmission line travels through the first optical fiber, then through the second optical fiber, then through the third optical fiber. The first, second and third optical fibers have first, second and third characteristic values, respectively, the second characteristic value being larger than the first characteristic value and the third characteristic value. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. The apparatus also includes a pump light source supplying pump light to the transmission line so that the signal light is amplified by Raman amplification as the signal light travels through at least one of the first, second and third optical fibers.